Laser Ignition of Levitated Char Particles

The temperature history of a char particle that was heated by a carbon dioxide laser to the ignition point was studied in the electrodynamic balance. The ignition characteristics of char particles are an important part of the overall coal combustion process, and the aim of research in this area is to improve efficiency and minimize waste products and pollution in current combustors. The experiments were conducted using 158-210 µm diameter Spherocarb particles, and 200-300 µm diameter char particles from a bituminous coal. Individual charged char particles were suspended in the electrodynamic balance and heated by a 420 ms pulse of radiation from a CO_2 laser at a heating rate of approximately 4.5 x 10^3 K/s. The temperature of the particle was measured with an optical pyrometer. The atmosphere in the electrodynamic balance was initially pure nitrogen. The oxygen concentration was slowly increased as successive laser pulses were applied, until the particle ignited. Upon ignition, the particle lost charge and dropped from the center of the balance. The particle temperature profile preceding ignition shows the ignition delay predicted by the general ignition condition of Semenov's thermal explosion theory. The measured temperature traces could be described well by calculations treating the particle as a sphere and using rate parameters given in the literature